In the production of a tubular preform, the gap of the outlet between the die core and the die body determines the wall thickness of the hollow body tube that is subsequently manufactured from the tubular preform in a blow mold device. If the hollow body is a technical molded part having a longitudinal axis deviating from a straight line (i.e., the molded part has at least one curve in the longitudinal direction), a preform extruded with a constant wall thickness over its circumference results in the molded part having a greater wall thickness (i.e., material accumulation) in the region of the smaller radius of curvature, as well as a thinner wall thickness in the region of the larger radius of curvature.
Typically, however, to ensure a tube of an appropriate strength, the wall thickness may not fall below a minimum wall thickness value at any point of the hollow body. To stay above this minimum value, it is necessary to consume more plastic material in the manufacture of the tube, thereby increasing the weight of the finished hollow body. In addition, in the event of small radii of curvature, the finished hollow body may have wrinkles on the inside and transverse marks on the outside in the region of the smaller radius of curvature as a result of the material accumulation, through which both the technical function (e.g., as an air conduction channel) and the appearance of the article suffer.
An extrusion head having die body displaceable by small amounts along a plane perpendicular to the longitudinal axis of the die core, wherein the head is configured to achieve a linear, stretched exit of the preform out of the outlet gap having a constant wall thickness around its circumference (and not a targeted setting of a varied wall thickness of the preform as a function of the circumference) is known from DE-A-21 28 901. For this purpose, the die body has a flange ring, positioned along the side facing away from the outlet gap, configured for transverse displacement between an upper and a lower retaining ring of the blow head. The flange ring is enclosed by a displacement ring, which is provided outside the extrusion blow head with two eyes (offset by 90° from each other) engaged by two pivotable arm levers connected via spindle drives. In this construction, great force is required to transversely displacement the die body in relation to the die core, because the force is introduced at the upper end of the die body, i.e., in a region in which the plastic melt is still under very high pressure (the plastic melt does exit out of the outlet gap practically without pressure, but has a pressure of 400 to 500 bar at the entry point into the blow head in current machines).
In addition, an extrusion blow head having a die body displaceable in two orthogonal directions using two eccentrically mounted displacement rings that enclose the die body, as well as levers engaging thereon, but is adapted to achieve a varied wall thickness of the preform as a function of the circumference is known from DE-C-195 37 132. This configuration, however, suffers from the same disadvantage discussed above, namely, it requires great force to displace the die body at a significant distance from the outlet gap.